Much attention has been directed to the development of packaging materials in a film, a semi-rigid or rigid sheet and a rigid container made of a thermoplastic composition. In such applications, the polymeric composition preferably acts as a barrier to the passage of a variety of liquid water or moisture vapor permeant compositions to prevent contact between, e.g., the contents of a package and water. Improving barrier properties is an important goal for manufacturers of film and thermoplastic resins.
Barrier properties arise from both the structure and the composition of the material. The order of the structure (i.e.,), the crystallinity or the amorphous nature of the material, the presence of adsorbents or absorbents in the material, the existence of layers or coatings can affect barrier properties. The barrier property of many materials can be increased by using liquid crystal or self-ordering molecular technology, by axially orienting materials such as an ethylene vinyl alcohol film, or by biaxially orienting polypropylene films and by using other useful structures. Internal polymeric structure can be crystallized or ordered in a way to increase the resistance to permeation of a permeant. A material can be selected, for the thermoplastic or packaging coating, which prevents absorption of a permeant onto the barrier surface. The material can also be selected to prevent the transport of the permeant through the barrier.
The permeation process can be described as a multistep event. First, collision of the permeant molecule with the polymer is followed by sorption into the polymer. Next, migration through the polymer matrix by random hops occurs and finally the desorption of the permeant from the polymer completes the process. The process occurs to eliminate an existing chemical concentration difference between the outside of the film and the inside of the package. Permeability of an organic molecule through a packaging film consists of two component parts, the diffusion rate and solubility of the molecule in the film. The diffusion rate measures how fast molecule transport occurs through the film. Solubility is a measure of the concentration of the permeant molecule that will be in position to migrate through the film. Diffusion and solubility are important measurements of a barrier film's performance. There are two types of mechanisms of mass transfer for organic vapors permeating through packaging films: capillary flow and activated diffusion. Capillary flow involves small molecules permeating through pinholes or highly porous media. This is of course an undesirable feature in a high barrier film. The second, called activated diffusion, consists of solubilization of the penetrants into an effectively non-porous film at the inflow surface, diffusion through the film under a concentration gradient (high concentration to low concentration), and release from the outflow surface at a lower concentration. In non-porous polymeric films, therefore, the mass transport of a penetrant includes three steps--sorption, diffusion, and desorption. Sorption and desorption depend upon the solubility of the penetrant in the film. The process of sorption of a vapor by a polymer can be considered to involve two stages: condensation of the vapor onto the polymer followed by solution of the condensed vapor into the polymer. For a thin-film polymer, permeation is the flow of a substance through a film under a permeant concentration gradient. The driving force for permeation is given as the pressure difference of the permeant across the film. Several factors determine the ability of a permeant molecule to permeate through a membrane: size, shape, and chemical nature of the permeant, physical and chemical properties of the polymer, and interactions between the permeant and the polymer. A permeant for this application means moisture vapor or water vapor. A typical barrier material comprises a single layer of polymer, a two layer coextruded or laminated polymer film, a coated monolayer, bilayer or multilayer film having one, or more coatings on a surface or both surfaces of the film or sheet.
The two most widely used barrier polymers for food packaging are olefin polymers such as polyethylene, polypropylene, ethylene-vinyl alcohol copolymers (EVOH) ethylene vinyl acetate copolymers (EVA) and polyvinylidene chloride (PVDC). Other useful thermoplastics include ethylene acrylic materials including ethylene acrylic acid, ethylene methacrylic acid, etc. Such polymers are available commercially and offer some resistance to permeation of gases, flavors, aromas, solvents and most chemicals. PVDC is also an excellent barrier to moisture while EVOH offers very good processability and permits substantial use of regrind materials. EVOH copolymer resins are commonly used in a wide variety of grades having varying ethylene concentrations. As the ethylene content is reduced, the barrier properties to gases, flavors and solvents increase. EVOH resins are commonly used in coextrusions with polyolefins, nylon or polyethylene terephthalate (PET) as a structural layer. Commercially, amorphous nylon resins are being promoted for monolayer bottles and films. Moderate barrier polymer materials such as monolayer polyethylene terephthalate, polymethyl pentene or polyvinyl chloride films are available.
The following table lists a water vapor transition rate (WVTR) for each of a variety of packaging polymers. An inspection of this table and a comparison to similar permeation rates for other permeants such as oxygen, hydrocarbons, carbon dioxide, etc. shows that different permeants have differing permeation rates in various polymers. A polymer that is a good oxygen barrier is often a poor water vapor barrier. Such a relationship can be qualitatively established considering the mechanism of transport through a barrier. Barrier polymers often rely on dipole-dipole interactions to reduce chain mobility and, hence, diffusional movement of permeants. These dipoles can be good sites for hydrogen bonding. Water molecules are attracted to these sites leading to a high solubility characteristic. Excellent examples of dipole-dipole interaction as a function of WVTR @ 37.80.degree. C. or 38.degree. C. and 90% RH.
______________________________________ POLYMER.sup.1 WVTR ______________________________________ Fluoropolymer 0.0119-0.0236 PVDC 0.02 HDPE 0.17 PP 0.4 EVOH (@ 40.degree. C.) 0.79-2.4 PET 0.39-1.7 ______________________________________ .sup.1 decreasing dipole to dipole.
Further, water molecules can enhance the diffusion by interrupting attractions and chain packing of the barrier material. Polymer molecules without substantial dipole-dipole interaction such as polyolefins, dissolve very little water and have low WVTR and other similar permeability values. The lower solubility more than compensate for the incrementally higher diffusion.
As expected, the solubility and diffusivity of liquids and gases in polymers are strongly dependent upon polymer molecular structure, chemical composition and polymer morphology. Properties related to solubility, such as permeability, also behave in a similar fashion. Accordingly, from solubility theory it is expected that the solubility of an organic penetrant in a polymer is related to the difference between the solubility parameter (.delta.) of both the penetrant and polymer. Good solubility is expected when the difference between solubility parameter values is close to a mean zero. It should be pointed out, however, that the solubility-parameter approach is useful only in the absence of strong polymer-penetrant interactions, such as hydrogen bonding.
The relationship between penetrant transfer characteristics and the basic molecular structure and chemical composition of a polymer is rather complex, and a number of factors contribute to the sorption and diffusion processes, among the most important being:
(a) structure regularity or chain symmetry, which can readily lead to a three-dimensional order of crystallinity. This is determined by the type of monomer(s) and the conditions of the polymerization reaction; PA1 (b) cohesive-energy density, which produces strong intermolecular bonds, Van der Waals or hydrogen bonds and regular, periodic arrangement of such groups; PA1 (c) chain alignment or orientation which allows laterally bonding groups to approach each other to the distance of best interaction, enhancing the tendency to form crystalline materials; and PA1 (d) the glass transition temperature (Tg) of the polymer, above which free vibration and rotational motion of polymer chains occur so that different conformations can be assumed.
Polymer free volume is also a function of structural regularity, orientation and cohesive energy density. The aforementioned structure-property relationships all contribute to a decrease in solubility and diffusivity, and thus permeability.
Examples of the effect of polymer molecular structure and chemical composition on the sorption equilibrium and diffusion values for acetone vapor by a series of barrier polymer films of varying functionality are shown in Table 1.
TABLE 1 ______________________________________ Water-vapor Transmission Rates of Selected Polymers.sup.a WVTR, Polymer nmol/(m.s) ______________________________________ vinylidene chloride copolymers 0.005-0.05 high density polyethylene (HDPE) 0.095 polypropylene 0.16 low density polyethylene (LDPE) 0.35 ethylene-vinyl alcohol, 44 mol % 0.35 ethylene.sup.b poly(ethylene terephthalate) (PET) 0.45 poly(vinyl chloride) (PVC) 0.55 ethylene-vinyl alcohol, 32 mol % 0.95 ethylene nylon-6,6 nylon-11 0.95 nitrile barrier resins 1.5 polystyrene 1.8 nylon-6 2.7 polycarbonate 2.8 nylon-12 15.9 ______________________________________ .sup.a At 38.degree. C. and 90% RH unless otherwise noted (13). .sup.b Measured at 40.degree. C. (KirkOthmer, Encyclopedia of Chemical Technology, p. 943, Volume 3 (4th Ed.), WileyInterscience)
An inspection of the table shows that a film made from these materials can have an appreciable moisture vapor transmission rate. However, in many applications improvement is needed. Contact between a packaged food, water sensitive material, or other item contained within the package with moisture vapor can result in a sensory change and/or spoilage. Control of moisture exchange with the environment, accomplished through barrier packaging, is crucial for moisture sensitive foods. Foods are complex biologically and chemically active systems that require strict control over their manufacturing, distribution and storage conditions in order to maintain a safety and sensory and nutritive values. Shelf life is the time period between processing and use during which a food stays within acceptable limits of quality. To ensure a high quality product for at least its targeted shelf life, environmental conditions such as temperature, moisture, gas composition and light must be controlled. Barrier packaging can be used to accomplish moisture control, gas composition and light effects. The primary function of the food package, besides serving as a containing unit, is to keep the food in a controlled microenvironment. The package itself becomes part of the food environment and the food package interactions have to be considered. The package can include an artificial atmosphere, can prevent light from penetrating and altering the food composition, and can maintain the water activity of the food constant or ensure that no additional moisture comes in contact with the food contents.
Water is of major importance in food preservation. Maintaining low water activity can act as an important preservation technique. Moisture content or activity (A.sub.w) can improve the texture quality, the tendency to lipid oxidation, the tendency for non-enzymatic browning, the enzymatic activity and the mold and bacteria growth. Water activity describes the degree of boundness of the water contained in the food and its availability to act as a solvent and participate in chemical or biochemical reactions. Critical levels of water activity can cause undesirable deterioration. From a safety standpoint, increased water activity can promote microbial growth. Textural qualities are also very important for consumers. Texture is greatly affected by water activity. Dry, crisp foods like potato chips, popcorn, crackers, corn flakes, and other breakfast or snack foods can quickly lose crispness and can become texturally unacceptable as moisture activity increases from about 0.3 to greater than 0.5 or more. Foods with intermediate moisture activity such as dried fruits, pet foods, bakery goods, etc. can become hard if water activity drops below about 0.6. Unpopped popcorn requires a water activity greater than about 0.3-0.5 to increase popping volume. Further, a number of the chemical constituents of foods can change from amorphous to crystalline or from crystalline to amorphous stage during storage in inappropriate water activities. Further, the ability of water to act as a solvent or reaction medium can change the nature and quality of the food product. Many reactions that promote food deterioration can increase in reaction rate as water activity increases. Clearly, water activity in foods is a significant feature. The ability to maintain foods within a certain water activity can be critical. Packaging should be able to prevent drying and prevent introduction of increased water activity into a food. The packaging industry has worked for many years to improve the barrier properties of a variety of packaging materials such as those shown in Table 1.
Substantial attention is now directed to a variety of technologies for the improvement of moisture vapor barrier properties. The use of both physical barriers and active chemical barriers or traps in packaging materials are under active investigation. In particular, attention has focused on use of specific copolymer and terpolymer materials, the use of specific polymer alloys, the use of improved coatings for barrier material such as silica, metals, organometallics, and other strategies.
Packaging scientists are continuing to develop new polymeric films, coated films, polymeric alloys, etc. using blends of materials to attain higher barrier properties. Many of these systems have attained some degree of utility but have failed to achieve substantial commercial success due to a variety of factors including obtaining barrier performance at low cost.
Yeh et al., U.S. Pat. No. 5,106,677 teach a water vapor barrier material comprising a styrene-butadiene polymer containing about 0.5-10% of a polyethylene wax as a barrier additive. Thomas et al., U.S. Pat. No. 5,378,510 disclose a barrier film made by forming a coating on the film using an organosilicon reagent in an oxygen atmosphere. Kunz et al., U.S. Pat. No. 5,387,449 teach a polymer laminate containing layers of polyester-ceramic materials or polyolefin materials. Deak et al., U.S. Pat. No. 5,085,904 teach a multilayer structure containing film materials such as polyester and barrier layers of silica materials. Lohwasser, U.S. Pat. No. 5,436,035 teach barrier materials comprising coated plastic films. The coatings are generated in a plasma phase and comprise ceramics such as silicon dioxide, silicon monoxide, aluminum oxide and others. Percec et al., U.S. Pat. No. 5,114,795 teach a multilayered barrier laminate film. Kuechler et al., U.S. Pat. No. 5,324,572 teach a multilayer laminate film having improved barrier properties. Sibbach et al., U.S. Pat. No. 4,912,103 teach a heat sealable barrier laminate film. Jorge et al., U.S. Pat. No. 5,073,617 teach an improved barrier film made by extrusion and then post-treating the film after extrusion to optimize barrier properties.
One problem that arises when searching for polymer blends or compounded polymeric materials, relates to the physical properties of the film. Films must retain substantial clarity, tensile strength, resistance to penetration, tear resistance, etc. to remain useful in packaging materials. Blending unlike materials into a thermoplastic before film extrusion often results in a substantial reduction of film properties. Finding compatible polymer materials for polymer alloys, and compatible additives for polymeric materials typically require empirical demonstration of compatibility and does not follow a clearly developed theory. However compatibility can be demonstrated by showing that the compounded material obtains an improved barrier quality with little reduction in clarity, processability, or structural properties using conventional test methods. Accordingly, a substantial need exists for development of materials that can be incorporated into polymeric material to form a packaging thermoplastic having excellent barrier properties without any substantial reduction in structural properties.